The present invention relates to a zoom lens having a large aperature or high zoom ratio with an inner focusing function. It especially relates to a zoom lens system having a relatively large lens i.e., having a large aperature and a relatively large field angle of at least 62.degree. and preferably over 75.degree..
Conventionally, most zoom lens systems which have been proposed are standard zoom lenses which comprise: in order from the object end,
a positive lens group; PA1 a negative lens group; and PA1 a positive lens group. PA1 a) a first lens group having a positive refractive power; and PA1 b) a second lens group having a negative refractive power; wherein the second lens group comprises, in order from the object end, PA1 y: is a height in a direction perpendicular to an optical axis; PA1 S (y): is a displacement amount along the optical axial direction at the height y (a sag amount: a distance along the optical axis between tangent planes at peaks of each aspherical surface); PA1 R: is a paraxial radius of curvature; PA1 K: is a conic constant; and PA1 C.sub.n : is a n-order aspherical surface constant.
More specifically, Japanese laid open patents S58-30709 and S58-179810 disclose a compact zoom lens system having a zoom ratio of about 3.times. and a maximum field angle 2.omega. of 62.degree..
In the related arts, a large aperature high zoom ratio lens system is proposed in the Japanese laid open patents H4-208911 and H4-208912. The high zoom ratio lens system disclosed in these patents has a maximum field angle 2.omega. of 62.degree., a zoom ratio of about 2.times., and the F number for the overall zoom range area is 2.8, which is large for a standard zoom lens system. In these high zoom ratio lens systems, focusing is performed by moving the second lens group and the third lens group.
Also, Japanese laid open patent S63-294,506 discloses a compact zoom lens system having a zoom ratio of about 3.times. and a maximum field angle 2.omega. of 76.degree.. Further, Japanese laid open H4-317,020 discloses a zoom lens system having an aspherical surface introduced to most of the lens surfaces which construct the second lens group.
The zoom lens disclosed in Japanese laid open patents S58-30709 and S58-17981 has an F number of 3.5, which is a relatively large aperature (relatively high speed). However, the zoom ratio is about 3.times. and the maximum field angle is only about 62.degree.. In terms of aberrations, the zoom lens does not correct spherical aberration. With the zoom lens, astigmatism fluctuates during zoom ratio. Coma aberration fluctuates as well. This, in turn, can be interpreted to mean that it is difficult to make a large aperature high zoom ratio lens having a field angle of over 75.degree. with a high zoom ratio and which is preferably able to correct for spherical aberrations. Note that focusing technique is not described in detail in these laid open patents.
With the large aperature zoom lens disclosed in Japanese laid open patents H4-208911 and H4-208912, aberrations are corrected efficiently. The F number is 2.8, which is high speed for a standard zoom lens system. However, the zoom ratio is about 3.times. and the maximum field angle 2.omega. is only 62.degree..
As described above, the zoom lenses of conventional technology are not perfect and a higher zoom ratio and wider field angle is demanded in the current technology. For the focusing technique, a technique which provides a constant aberration and focusing with a simple mechanism, is desired.
The zoom lens system disclosed in Japanese laid open patent S63-294506 has an F number of 3.5 to 4.5, which is a small aperature. Coma aberration fluctuates as well. Therefore, with this optical system configuration, it is very difficult to obtain a large aperature and wide-angle zoom lens system which can provide an F number of over 2.8 throughout the zoom range.
The zoom lens system disclosed in Japanese laid open patent H4-317020 has an aspherical surface. However, the aspherical surface is introduced to reduce the size and cost of the lens system. In view of allowing increased aperature size and wide-angle for a high zoom ratio lens system, the way the aspherical surface is used and the way aberrations are corrected are fundamentally different from the present invention. As a result, the zoom lens system has an F number of 4 to 5.8, which is a small aperature. In terms of aberration correction, spherical aberration and stigmatism still fluctuate during zooming. Therefore, with this optical system configuration, it is very difficult to obtain a large aperature and wide-angle zoom lens system which can provide an F number of over 2.8 throughout the zoom range.
In the large aperature zoom lens system disclosed in Japanese laid open patents H4-208911 and H4-208912, aberrations are corrected very well. The F number is 2.8, which is high speed but the maximum field angle is about 62.degree., which is small. As a result, a larger field angle has been desired for the zoom lens system of conventional technology.
The present invention intends to resolve the above problem. It intends to provide a high zoom ratio large aperature zoom lens system with a large field angle, a zoom ratio of over 2.6.times., and an F number of about 2.8.
The present invention also intends to provide a high zoom ratio lens system with a large aperature (e.g. F number: 2.8 to 3.5) which can provide a zoom ratio of over 3.5.times..